mono-grade oil requirement DD two stroke diesels? 1

[Bevans6]

I also posted this on the Engine forum, since this one seems to see little activity. I hope that's OK.

I'm trying to find out the root reason why Detroit Diesel two stroke engines require, to this day, mono-grade engine oil, normally straight 40 weight. Empirical evidience, supported by recommendations from DD, is that the engines last longer between rebuilds using straight weight oil in normal service. I just don't understand why. what is there about a two stroke application that would make a multi-grade oil, of completely modern and high quality manufacture, less protective in this application?

The "common knowledge" answer seems to be that multi-grades revert to their cold viscosity under the pressure of two stroke compression cycles, that the hydrodynamic layer of oil in the bearings can't recover between power strokes, or that the long-chain molecules used in viscosity modifiers shear and break down. I just have a hard time lend credence to how a two stroke diesel, with 18:1 compression, running at 2100 rpm is that different from some other high duty engine requirement.

Any ideas?

Thanks for looking, Brian

 

[Bribyk]

Most manufacturers of medium to very large industrial natural gas engines also recommend a monograde (30 or 40) oil, except for cold starting applications. At least one manufacturer sites viscoity reduction due to shearing as the reason.

Brian Bobyk - Hoerbiger Canada

 

[romke]

the reason was fourfold.

first the fact that the original multigrade oils did not have very stable viscosity index improvers - they broke down quite fast from the original SAE 20W-40 to SAE 10W-30, resulting in lower wear protection then anticipated. nowadays that problem has completely been overcome.

the second reason was temporary shear, as already mentioned. that problem has also largely been overcome.

a third reason was that the use of viscosity index improvers in DD engines tended to give more deposits in the pistonring area. nowadays it would be possible to use better quality baseoils that require less viscosity index improver so that problem can be solved.

finally the fourth reason was that conventional diesel engine oil contained quite a amount of metallic additives that also contributed to deposits in the ringbeltarea of DD two stroke engines.

DD therefore required a singlegrade engine oil with a restriction on the sulphated ash content. that requirement i reflected in the API CF-2 classification

it all boils down to the fact that DD engines have a different "oil appetite" then fourstroke engines.

that is also reflected in the contruction details of the engines: cast iron pistons are used because aluminium based pistons where not able to withstand the local heat induced stresses.

 

[drwebb]

Note that several reputable CJ-4 products also claim CF-2 performance, further supporting deposits and cleanliness requirements not being mutually exclusive between 4T and 2T diesel oils anymore.

 

[rmw]

"Oil appetite" is right; gluttons they were (are). Doesn't their manual say that their rated oil consumption is one oil refinery in their lifetime?

rmw